1. Field of the Invention
The invention relates to a zoom lens and an image pickup apparatus equipped with the zoom lens, and more particularly, to a zoom lens suitable for a photographic optical system used in an image pickup apparatus such as a video camera, a digital camera, a broadcasting camera, a monitoring camera, or a silver-halide film camera.
2. Description of the Related Art
In recent years, image pickup apparatuses such as video cameras, photographing digital cameras, broadcasting cameras using a solid-state image sensor, and silver-halide film based cameras have been miniaturized while maintaining high functionality. To that end, there has been proposed an image pickup apparatus in which the entire zoom lens is miniaturized without using a quick-return mirror as a monocular interchangeable lens system. In addition, there is a demand for a photographic optical system, in which the entire lens length (a distance from the first lens surface to the image plane) is short, the front lens effective diameter is small, the entire zoom lens is compact, and the zoom lens has a large diameter and a wide angle of view.
In the current state of the art, there is known a four-unit zoom lens including a first lens unit of a positive refractive power, a second lens unit of a negative refractive power, a third lens unit of a positive refractive power, and a fourth lens unit of a positive refractive power in order from an object side to an image side, where each lens unit is moved during zooming. Japanese Patent Application Laid-Open No. 08-271790 and U.S. Patent Application Publication No. 2005/0041304 both disclose a zoom lens in which an aperture stop is arranged on the image side of the third lens unit so that the entire zoom lens is miniaturized.
In addition, there is known a zoom lens having an image stabilizing function in which a part of the lens units are displaced to a direction perpendicular to an optical axis to correct an image shake. U.S. Pat. No. 7,782,544 discusses a zoom lens in which an image shake is corrected by moving the entire third lens unit out of the four-unit zoom lens to a direction perpendicular to an optical axis to obtain a stationary image.
In spite of the current state of the art, a demand exits for a zoom lens having a small outer diameter of the lens barrel, a miniaturized size in entirety, and a large aperture ratio. In general, in order to implement a large aperture ratio and a miniaturized size of the entire zoom lens, it is necessary to increase refractive powers of each lens unit of the zoom lens and reduce movement amounts of each lens unit during zooming. However, in the zoom lens configured in such a manner, as the refractive powers of each lens surface increase, the lens thickness also increases. Therefore, an effect of reducing the lens system is insufficient, and it is difficult to correct various kinds of aberration.
In the four-unit zoom lens described above, it is important to appropriately set the refractive powers of each lens unit, a position of the aperture stop, a lens configuration of the first and third lens units, and the like in order to obtain both the miniaturized size in the entire lens system and the large aperture ratio. If the position of the aperture stop or the lens configuration of the lens unit arranged in the vicinity of the aperture stop, for example, the third lens is not appropriate, it is difficult to obtain a large aperture ratio, a reduced front lens effective diameter, a reduced lens barrel diameter, and a miniaturized size of the entire zoom lens.